Mercury arc lamps have a number of applications in industry such as ultraviolet lamps for drying ink in printing applications. Industrial applications often require that the output from the lamp be controlled.
An example of such an application is illustrated schematically in FIG. 1, which represents an ultraviolet curing system for a printing application. After applying UV inks or coatings (2), a substrate (1) passes under an ultraviolet lamp (3) causing the monomers within the ink or coating to cross-link and cure. On certain applications the substrate will stop underneath the ultraviolet lamp (3) which is controlled to switch down to 20–30% of its nominal power. However, on recently developed heat-sensitive substrates (1) this level of power can still be sufficient to cause the material (1) to melt or burn.
The power output of a lamp is typically controlled by switching capacitors into and out of the lamp circuit as described, for example, in U.S. Pat. No. 4,873,470. The practical limits of this arrangement are about 20% of normal full power. Any further reduction in lamp power results in the lamp's operation becoming unstable, for example the lamp flickers, which is undesirable for both the curing operation to which the lamp is applied and the lamp life.